Standard computer monitors and TV displays are typically based on three additive primaries; namely, red, green and blue, collectively denoted RGB. These monitors may not be able to display many colors perceived by humans, since they are limited in the range of colors they are capable of displaying.
Existing display devices can be divided into two groups, namely, direct view devices and projection devices. The direct view devices include CRT, LCD, LED and other types of display. In direct view devices, a display screen is composed of a plurality of, for example, RGB pixels, each pixel including a red sub-pixel element, a green sub-pixel element and a blue sub-pixel element. The color image is created by the viewer's visual system, which mentally integrates the colored light awing from spatially neighboring sub-pixels to give a full color impression.
Projection display systems create images by projecting light on a viewing screen There are generally two types of projection display systems, namely, simultaneous displays and sequential displays. Simultaneous projection display systems are based on projecting light of all primaries (e.g., three primaries) simultaneously onto to the viewing screen, whereby color combinations are perceived by spatial integration of the colors by the visual system of the viewer. Sequential projection display systems project separate images of the different primary colors onto the screen sequentially, at a sufficiently high frequency so that the human eye can perceive color combinations by temporal integration of the primary color images.
There are various types of Spatial Light Modulators (SLMs) for creating the patterns used to form the images displayed by projection display systems. For example, Liquid Crystal on Silicon (LCoS) devices or Digital Micromirror Devices (DMD™) may be used to create pixilated images for sequential projection devices. In designs of “optical engines” for producing the images displayed by projection display systems, different configurations of reflective, refractive, polarizing and filtering optical elements may be used in conjunction with at least one SLM.
A cross-sectional top-view of an optical engine of a prior art reflective LCoS projection display device, using a single LCoS panel, is illustrated schematically in FIG. 1. A device of this type is described, for example, in Edward H. Stupp, Mathew S. Brennesholtz, Projection Displays, John Wiley and Sons, 1999, the disclosure of which is incorporated herein by reference.
The device includes an illumination unit 101, consisting of one or more light sources, e.g., a plurality of high energy arc-lamps, typically high pressure mercury lamps as are known in the art, and beam shaping optics, e.g., integrating tunnel or lens-array type shaping optics as are known in the art. The light passes through a color wheel 102, which includes typically three, sequentially disposed, primary color filter segments, e.g., RGB filter segments, to produce a sequence of primary color image components. Color wheel 102 may be rotated, for example, by a rotation mechanism 112. In additional to the three primary color filter segments, color wheel 102 may optionally include a neutral filter segment, which may enhance the over-all illumination intensity of the display by providing periods of white-light illumination. The filtered light from color wheel 102 is transmitted, through a relay lens 103 and a Polarizing Beam-Splitter (PBS) 104, onto a LCoS panel 105. As is known in the art, only a p-polarization component of the imaging light is transmitted through PBS 104. LCoS panel 105 includes an array of pixel elements, which are selectively modulated, by driving electronics, to produce a sequence of patterns corresponding to a sequence of primary color image components, which are temporally integrated by the viewer to form the desired color image. Each pixel of he LCoS panel 105, when activated to an “on” state, converts the p-polarized light into corresponding s-polarized light, as is known in the art, and reflects the converted light back towards PBS 104. The converted s-polarized light is then reflected by PBS 104 onto a projection lens 106, or arrangement of lenses, which focuses the light on a display screen, e.g., a reflective display screen or diffusive (i.e., back-illuminated) display screen.
A drawback of the above configuration is the inherent loss of “second polarization” light, i.e., the s-polarization component of the original light transmitted through lens 103 and rejected by PBS 104. This loss of power can be reduced by implementing conversion techniques, as are known in the art, within illumination unit 101, for example, mechanical arrangements of polarizing filters, prisms and reflectors that convert some of the p-polarized light into s-polarized light; however, only up to about 50 percent of the lost light can be recovered using such elaborate techniques, due to systemic inefficiencies of the optical configurations used in implementing such techniques.
An optical engine including two DMD™ SLM panels has been used by the prior art to compensate for low intensity output in the red wavelength in early model high-pressure lamps. Such a two-panel configuration is described in detail in Edward H. Stupp, Mathew S. Brennesholtz, “Projection Displays”, John Wiley and Sons, 1999 (“Stupp-Brennesholtz”), the disclosure of which is incorporated herein by reference. In this configuration, light from an illumination unit passes through a two-segment color wheel, wherein the two color segments are yellow and magenta. A color splitting prism directs either blue light (when the color wheel is in the “magenta” position) or green light (when the color wheel is in the “yellow” position) to a first DMD™ panel, which modulates the blue and green color components of the image. In both positions of the color wheel, red light is directed to a second DMD™ panel, which modulates the red color component of the image. The reflected illumination from both DMD™ panels is merged and projected through a projection lens, or arrangement of lenses. In this arrangement, the red light output deficiency of some early model white light sources is overcome by effectively doubling the display time of the red light component of the image. The color gamut of the resultant image is within the confines of the conventional RGB color gamut.
U.S. Pat. No. 6,280,034 (“the '034 patent”), the disclosure of which is incorporated herein by reference describes an imaging system including an illumination unit, which has a broadband non-polarized white light source, and a polarization converter system (PCS): which converts the non-polarized light into polarized light of substantially a single polarization axis. The system of the '034 patent further includes a selective polarization filter which, based on control signals, rotates the polarization axis of a selected spectral band of the white light with respect to the remaining (i.e., complementary) spectral bands, producing two, complementary, orthogonal, polarized spectral components. The two spectral components are divided into separate light beams, using a polarized beam splitter (PBS), and each beam is separately modulated, using a spatial light modulator (SLM), to produce a desired image pattern. Using an additional PBS, the two image patterns are re-combined and projected onto a viewing screen. The selective polarization filter may include more than one region, e.g., a red filter region and a blue filter region, which may be used sequentially to produce a time-division-multiplexed color image. It should be noted that, in the configuration of the '034 patent, the intensity of the polarized light produced by the PCS does not recover the full intensity of the non-polarized light generated by the illumination unit; rather, only up to 30–60 percent of the lost light intensity may be recovered. Additionally, the selective polarization method of the '034 patent produces inherently complementary colored light beams, e.g., a red beam and a cyan beam, or a blue beam and a yellow beam. Consequently, the two separate light channels simultaneously modulated by the two SLMs of the '034 patent are co-dependent, in the sense that the colors of the two channels are inherently complementary. Therefore, although the arrangement of the '034 patent may improve image brightness, the inherent co-dependency of the two channels limits the color gamut that can be produced by the system to the confines of a conventional RGB color gamut.